1. Technical Field
The present invention relates to a projection lens system of a projector that enlarges and projects an image displayed by a light modulator, such as a liquid crystal panel or DMD, onto a screen.
2. Description of the Related Art
Japanese Laid-Open Patent Publication No. 2002-31754 discloses a projection-type image display apparatus that modulates light from a light source in pixel units by transmitting the light using a liquid crystal display device or reflecting the light using a micromirror device (DMD) and then enlarging and projecting the modulated light using a projection lens apparatus so as to produce a visual image.
The projection lens apparatus disclosed by Japanese Laid-Open Patent Publication No. 2002-31754 includes an arrangement of a first lens group with a negative refractive power, a second lens group with a positive refractive power, and a third lens group with a positive refractive power that are disposed in this order from a screen-side towards an image display element side. The publication states that by using aspherical lenses in the first lens group, the second lens group, and the third lens group, distortion and chromatic aberration of magnification are corrected by the first and third lens groups and aberration of rays on the optical axis is corrected by the second lens group, so that it is possible to favorably control various aberrations while reducing apparatus size and widening the projection angle.
A retrofocus-type projection lens system having a first lens group with a negative refractive power and second and third lens groups with a positive refractive power can construct a telecentric optical system that is wide angle and has a long back focus, and so is suited to a projection lens system of a rear projector such as that shown in FIG. 1. However, when a lens system is made wide-angle, it becomes difficult to correct distortion, curvature of field, and astigmatism. In particular, it is problematic to correct the distortion, curvature of field, and spherical aberration for a retrofocus lens system for a wide-angle lens. Accordingly, the projection lens apparatus disclosed by Japanese Laid-Open Patent Publication No. 2002-31754, attempts to favorably correct aberration by using aspherical lenses in the first lens group, the second lens group, and the third lens group, respectively.
By using aspherical lenses, aberration can be favorably corrected using a low number of lenses. However, there are the problems that aspherical lenses are expensive and are easily affected by minute expansions and/or contractions in the lens surfaces due to changes in temperature. Additionally, aberration performance is greatly affected by the precision of the lens surfaces during manufacturing. Accordingly, increasing the number of aspherical lenses makes a lens system costly. At the same time, when fluctuations in the aberration performance of a plurality of aspherical lenses cannot be cancelled out, such a lens system will be easily affected by temperature changes and/or a performance of such a lens system becomes fluctuate.
In a rear projector such as that shown in FIG. 1, the projection lens system is disposed in a completely closed environment, and compared to a conventional screen and projector system where at least part of the lens system is exposed, such projection lens system is susceptible to the effects of heat. The rear projector 1 shown in FIG. 1 includes, inside a housing 2, a light source 3, a light modulator (light valve) 4 that modulates light from the light source according to an image signal to form an image, a projection lens system 5 that projects projected light 8 from the light valve 4 onto a screen 9 via a rear surface, and mirrors 6 and 7 that guide the projected light 8 to the screen 9. A CRT, in which the light source 3 and the light valve 4 are integrated, can be also used. In recent years liquid crystal panels have often been utilized as the light valve 4, and DMD panels, formed of the micromirrors mentioned above, are also utilized. In the case of a DMD panel or a reflective type LCD, the light valve 4 is a reflective-type, so that the positional relationship with respect to the light source 3 differs to that shown in FIG. 1. Anyway the input side of the projection lens system 5 will be telecentric regardless of whether a liquid crystal panel or a DMD panel is used as the light valve 4. Therefore, a projection lens system 5 that input side is telecentric and enlarges and projects the modulated light (projected light) 8 is required.
It is an object of the present invention to provide a projection lens system whose input side is telecentric, which enlarges and projects modulated light, and has a similar or higher aberration correction performance in spite of making the greatest possible reduction in the number of aspherical lenses.